The two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO), in Hanford (WA) and Livingston (LA), and the Virgo detector, near Pisa, Italy, have detected gravitational waves from colliding neutron stars for the first time. The two stars were 10 percent to 60 percent more massive than our Sun. This cosmic firework occurred in the galaxy NGC4993, which is about 130 million light years away. In a path-breaking achievement various forms of electromagnetic radiation from the collision were observed by multiple telescopes, many of which were alerted by LIGO and Virgo soon after their detection on August 17, thereby launching the era of multi-messenger astronomy with gravitational waves.
Earlier this month Rainer Weiss, Barry Barish and Kip Thorne were awarded the Nobel Prize for Physics for the direct observation of gravitational waves by LIGO, in 2015. Beginning with the discovery of the first binary black hole merger, christened GW150914, three other black hole mergers have been detected. The binary neutron star merger is the fifth LIGO detection, and the second one involving the Virgo detector. Neutron stars are the smallest, densest stars known to exist and are formed from the cores of massive stars when they explode in supernovae. Neutron stars are usually heavier than the sun, but have a much smaller diameter, of about 20 kilometers.
The binary neutron star detection is described in a recent paper in the journal Physical Review Letters and has been named GW170817. It was made by scientists and engineers of the LIGO Scientific Collaboration and Virgo Collaboration. Washington State University professor Sukanta Bose, graduate student Bernard Hall, and adjunct faculty member Nairwita Mazumder contributed to this finding.
The three WSU scientists collaborated with other members of the LIGO Scientific Collaboration in multiple ways to make this unique discovery. One of them was to characterize the detectors in order to improve their sensitivity to these weak gravitational-wave signals. Occasionally disturbances in a detector or its surroundings create noise artifacts that can potentially masquerade as gravitational-wave signals or obscure them. Over the years the collaboration has developed a set of analyses that can discern the former from the latter. Hall, Mazumder, and Bose contributed to the development of a couple of methods that extend this set in a new direction. Hall applied one of them to the detector data around the time of GW170817 and helped rule out an important type of noise artifact as its possible cause. He also happened to be on shift on the night before GW170817 occurred watching for possible problems in the equipment during observations.
Bose, who authored a paper in May on how the emissions from the remnant of neutron star mergers can provide a window onto the densest state of matter, contributed to the analysis of the merger energetics.
That work from May is cited in the GW170917 paper. His early work on how to establish whether or not signals in multiple detectors located around the globe originated from the same astrophysical source is an oft-cited paper; that work also showed how such signals can be combined to improve the localization of their source in the sky. Another of his recent work, on how to strategically point telescopes to find electromagnetic counterparts to gravitational wave sources, was adapted for observations by the Very Large Array radio telescope in New Mexico, which successfully observed radio emission from the merger.
“While GW170817 offers vital clues on the densest form of matter in the universe, we wait in anticipation of observations from more events to be able to understand it more precisely,” said Bose. He went on to add that combining observations of gravitational and electromagnetic waves was instrumental in multiple findings. One of these is proving the conjecture that neutron star collisions are indeed the source of one of the most energetic events in the cosmos, known for a few decades as short gamma ray bursts. The other is a new way of establishing the rate at which the Universe is expanding.
LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSFwith Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia(Australian Research Council) making significant commitments and contributions to the project.
More than 1,200 scientists from around the world participate in the effort through the LIGO ScientificCollaboration, which includes the GEO Collaboration.
The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 differentEuropean research groups: six from Centre National de la Recherche Scientifique (CNRS) in France;eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; theMTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; andthe European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.